This invention relates to a process for preparing a diorganodichlorosilane through disproportionation reaction of organochlorosilanes.
In the industry, organochlorosilanes are generally produced by direct synthesis known as Rochow method. This reaction yields diorganodichlorosilane of the most interest as a main product and by-products such as organotrichloro silane, triorganochlorosilane, and organodichlorosilane which are less useful and superfluous. There are additionally formed low-boiling fractions and high-boiling fractions such as disilanes, which must be discarded in a substantial sense.
As a solution to this problem, U.S. Pat. Nos. 2,647,136, 2,647,912, 4,447,631 and 4,552,973 propose to convert less useful organochlorosilanes to more useful organochloro silanes in the presence of a Lewis acid or a Lewis acid and a SiH group-containing compound. The most difficulty with these methods resides in the sublimation of AlC3 used as the Lewis acid. Due to a low reaction rate, reaction must be effected at high temperature and high pressure for a long time, raising a safety problem.
Therefore, an object of the present invention is to provide a process for preparing a useful diorganodichloro silane in a high yield from less useful organochlorosilanes through reaction at a low temperature and a high rate.
It has been found that when an organochlorosilane is subjected to disproportionation reaction in the co-presence of a compound having a hydrogen atom directly bonded to a silicon atom and in the presence of a primary catalyst of AlCl3 or AlBr3 and a co-catalyst of a specific metal or metal compound, this reaction takes place at a high rate even at atmospheric pressure and a low temperature (at which sublimation of AlCl3 or AlBr3 gives rise to no substantial problem), thereby yielding a useful diorganodichlorosilane.
The present invention provides a process for preparing a diorganodichlorosilane, comprising the step of subjecting an organochlorosilane to disproportionation reaction in the co-presence of a compound having a hydrogen atom directly bonded to a silicon atom and in the presence of a primary catalyst of AlC3 or AlBr3 and a co-catalyst selected from the group consisting of Mg, Al, Ca, Ti, Fe, Ni, Cu, Zn and Sn, compounds of these metals excluding AlCl3 and AlBr3, and mixtures thereof.
The process of the present invention starts with organochlorosilanes which are silane compounds having an organic group and/or a chlorine atom bonded to a silicon atom, for example, monoorganotrichlorosilanes, triorgano monochlorosilanes, tetrachlorosilane, tetraorganosilanes, and disilanes.
The organochlorosilanes may be used alone or in admixture of two or more. Inclusion of a diorganodichloro silane in the organochlorosilane mixture is acceptable. It is preferred to use an organotrichlorosilane such as methyltrichlorosilane and a triorganochlorosilane such as trimethylchlorosilane as the starting charge. The proportion of these organochlorosilanes in the starting charge is not critical because the invention is also directly applicable to crude silanes. When organochloro silanes in the starting charge are selected and adjusted, a proper choice is made on a stoichiometric basis so as to produce a maximum amount of diorganodichlorosilane.
The organic groups on the organochlorosilanes are preferably monovalent hydrocarbon groups having 1 to 6 carbon atoms, for example, alkyl groups such as methyl and ethyl and aryl groups such as phenyl. Of these, methyl, ethyl and phenyl are preferred, with methyl being most preferred.
The compound having a hydrogen atom directly bonded to a silicon atom, which is referred to as SiH group-containing compound, hereinafter, is not critical as long as it has a SiH group. Preferably it is an organosilane compound having the following general formula:
RaHbSiCl4-a-b 
wherein R is a monovalent hydrocarbon group preferably of 1 to 6 carbon atoms, more preferably methyl, ethyl or phenyl, most preferably methyl, xe2x80x9caxe2x80x9d is an integer of 0 to 3, xe2x80x9cbxe2x80x9d is an integer of 1 to 4, and a+b is an integer of 1 to 4. Illustrative are methyldichlorosilane and dimethylchloro silane, with methyldichlorosilane being most preferred.
The SiH group-containing compounds may be used alone or in admixture of two or more. An appropriate amount of the SiH group-containing compound used is 0.01 to 50 parts, more preferably 0.1 to 30 parts, most preferably 1 to 25 parts by weight per 100 parts by weight of the organochloro silane charge.
It is noted that the organochlorosilane and the SiH group-containing compound may be individually added to the reaction system, although a mixture of organochlorosilanes etc. Resulting from the manufacture of organochlorosilane (typically by Rochow method) may be used.
In the present invention, AlCl3 or AlBr3 is used as the primary catalyst, with AlC3 being most preferred. An appropriate amount of the primary catalyst used is 0.01 to 50 parts, more preferably 0.1 to 30 parts, most preferably 1 to 25 parts by weight per 100 parts by weight of the organochlorosilane charge.
In addition to the primary catalyst, the present invention uses a co-catalyst selected from among the metals Mg, Al, Ca, Ti, Fe, Ni, Cu, Zn and Sn, compounds of these metals excluding AlCl3 and AlBr3, and mixtures thereof. The metal compounds include alloys, oxides, hydroxides, chlorides, carbonates, and sulfates. Exemplary of the metal compounds are magnesium oxide, aluminum oxide, calcium oxide, titanium oxide, iron oxide, nickel oxide, copper oxide, zinc oxide, tin oxide, calcium hydroxide, magnesium chloride, zinc chloride, magnesium carbonate, magnesium sulfate, and brass. Of these, magnesium, magnesium oxide, magnesium chloride, zinc oxide, calcium oxide, calcium hydroxide, zinc chloride and brass are preferred, with magnesium oxide being most preferred.
An appropriate amount of the co-catalyst used is 0.1 to 20 parts, more preferably 1 to 10 parts by weight per 100 parts by weight of the organochlorosilane charge and 1 to 50 parts, more preferably 2 to 30 parts by weight per 100 parts by weight of the primary catalyst. Outside the range, a smaller amount of the co-catalyst may fail to achieve the desired effect whereas a larger amount of the co-catalyst may give a reaction system which contains too much solid components and is difficult to handle.
In the practice of the invention, disproportionation reaction may be carried out on the organochlorosilane charge in the presence of the aforementioned components. In a preferred embodiment using a mixture of methyltrichloro silane and trimethylchlorosilane as the organochlorosilane, dimethyldichlorosilane which is most useful in the silicone manufacture can be produced in high yields.
The reaction may be carried out in any desired way, typically in a well-known way, for example, by mixing the aforementioned components in a reactor and conducting reaction while stirring. The reaction temperature is preferably a temperature at which no substantial sublimation of AlCl3 or the like occurs, for example, 100xc2x0 C. or lower, and especially 80xc2x0 C. or lower. The lower limit of reaction temperature is not critical and preferably at or above room temperature (e.g., 25xc2x0 C.). With respect to the pressure, the reaction takes place even under atmospheric pressure although some pressure may be applied for accelerating the reaction.